The existence of a static electric field in the earth's atmosphere has long been recognized. More recently, this static electric field was determined to exhibit an essentially vertical gradient, the change in potential per meter decreasing with increasing altitude. Thus, at any altitude, equipotential lines and surfaces are essentially horizontal, especially in a localized area, subject to protrusions on the earth's surface. The ability to define such a horizontal equipotential line or surface in the atmosphere provides a reference for aircraft stabilization, gyroscopic drift correction, and clear air turbulence detection. Additionally, detection of tilted equipotential lines or surfaces in the vicinity of various physical objects provides a reference for obstacle avoidance apparatus.
In particular, the present invention provides methods for utilizing the earth's static field to obtain a reference signal which can be employed to control both pitch and roll axes stabilization of airborne vehicles. In a general sense, the invention provides for continuously defining an equipotential line or surface within the earth's generally vertical static potential gradient. Definition of this equipotential line or surface is indicated by DC voltage signals produced by apparatus associated with the invention, these signals being usable to accomplish a variety of functions. One method of the invention comprises immersing at least two voltage probes or the like, which sense potential magnitude, in the earth's static potential gradient and measuring the difference sensed by the two probes. If the two probes sense the same potential, i.e., the difference between the voltages generated at each probe is zero, then the probes lie on a line or surface of equal potential, usually horizontal due to the vertical nature of the earth's static potential gradient. Similarly, an equipotential plane may be defined by two pairs of voltage sensing probes or by three probes, one of which is referenced to the electrical center or some other point located between the other two probes. Even where the vertical gradient is distorted due to physical objects or ionization sources, nonhorizontal equipotential lines and surfaces may be sensed by the present invention.
Alternatively, the angle of misalignment between a line or plane containing two points in space or three noncolinear points in space, respectively, and an intersecting equipotential line or surface, respectively, can be readily determined without specific resort to roll or pitch measurements. One technique which may be used provides for determining the field vector associated with the point-containing line or plane. When the field vector is perpendicular to the associated line or plane, such line or plane is on the equipotential. A conventional electrostatic fluxmeter or field mill as described in Atomspheric Electricity by Chalmers (2nd Edition), pps. 143-145, can perform this function. As an alternative to measuring vector angle directly, a conventional vector resolver can be used to determine the magnitude of the field components along the line or in the plane defined by the points in space, viz. the plane of the aircraft. When such field components are zero, the field vector is perpendicular and the points are of equipotential. The angle between the field vector and the perpendicular, which can be found readily by conventional techniques, represents the misalignment angle (e.g. angle of banking, climbing, or diving) of an aircraft that contains the two or three points in space. A signal proportional to or related to the angle can be generated and fed to a servomechanism which can adjust the aircraft wings or fins to drive the minimal angle to zero (or some other angle), if desired, aligning the plane of the aircraft with an equipotential surface or as otherwise desired to control aircraft orientation.
One apparatus associated with the present invention comprises sensing probes (which probes may be comprised of radioactive material in order to increase the electrical contact thereof with the atmosphere) and a differential static amplifier. The amplifier receives the sensed potentials from the probes and measures the difference in the sensed potentials to produce a voltage difference signal which may be utilized to control a servomechanical or other apparatus. The voltage sensing probe/differential static amplifier combination is referred to hereinafter as a differential static voltmeter. The actual or effective combination of two differential static voltmeters can essentially define an equipotential surface in the earth's vertical static potential gradient. Since this equipotential surface is defined within a relatively small space, the surface is essentially flat and horizontal and may be used as a reference plane for aircraft stabilization.
If the sensing probes described above are attached to the wingtips of an aircraft or essentially along the transverse axis thereof, then the voltage difference between the probes measured by the differential static amplifier is a function of the roll angle of the aircraft. Similarly, the voltage difference sensed by a pair of sensing probes disposed essentially along the longitudinal axis of an aircraft is a function of the pitch angle of the aircraft. The differential voltage output signals thus produced for the pitch and roll orientations of the aircraft can be used to drive a servomechanical system to maintain the aircraft in level flight relative to the surface of the earth. The sensing probes need not be exactly aligned with or parallel to the transverse or longitudinal axes of the aircraft in order to produce useful information. Similarly, the probes need not be positioned at exactly the same elevation relative either to the earth's surface or to the horizontal axis of the aircraft.
In an embodiment where the probes are replaced by an electrostatic fluxmeter, field mill, or the like a similar servomechanical system can be utilized to adjust or measure the orientation of the aircraft. With the fluxmeter, the direction of the field vector can be derived relative to a line (such as wing-to-wing or nose-to-tail) of or to the plane of the aircraft, or, alternatively, the normal to such line or plane. The orientation of the aircraft can be changed to place the vector in a desired direction with respect to the normal to the plane of the aircraft. If the field vector is on the normal, the field component in the plane of the aircraft is zero and the aircraft is on an equipotential. By measuring the angle of the field vector with respect to the normal, the angle of misalignment is readily determined. By measuring the field component in the plane (or along a selected line) of the aircraft, a quantitative determination of the magnitude of misalignment results. Using either or both measures, the misalignment can be converted into a signal or signals which may be used in aircraft orientation, stabilization, or the like. Due to the customary roll and pitch control elements on the aircraft the system may, as in the voltage probe embodiment, derive separate signals for the separate control elements.
The invention, as discussed, provides for the measuring of the angle between a reference equipotential surface and a plane containing at least three noncolinear points in space at which sensors are located.
The invention also relates to a method and apparatus for measuring the roll rate or pitch rate of an airborne vehicle. The apparatus used to sense rate comprises a pair of conductive wires disposed essentially along or parallel to the transverse axis or longitudinal axis of the aircraft. Rotation of these wires in the earth's static electric field generates a current in the wires. The generated current is measured by a current meter, the intensity of the current being proportional to the roll rate or pitch rate of the vehicle.
Thus, it is an object of the invention to provide a method and apparatus for continuously defining an equipotential line or plane in the earth's static electric field relative to a given line or plane.
It is another object of the invention to provide differential voltage sensing apparatus aboard an airborne vehicle, the differential voltage sensed by said apparatus being a function of the roll or pitch attitude angle of the vehicle.
It is a further object of the invention to provide a differential static current meter for determining the roll rate or pitch rate of an airborne vehicle by measuring the current induced in a pair of conductive wires rotated in the earth's static potential gradient.
Further objects and advantages of the invention will become more readily apparent in light of the following detailed description.